In the publication Design of a MEMS Discretized Hyperbolic Paraboloid Geometry Ultrasonic Sensor Microarray, IEEE Transactions On Ultrasonics, Ferroelectrics, And Frequency Control, Vol. 55, No. 6, June 2008, the disclosure of which is hereby incorporated herein by reference, the inventor describes a concept of a discretized hyperbolic paraboloid geometry beam forming array of capacitive micromachined ultrasonic transducers (CMUT) which is assembled on a microfabricated tiered geometry.
In initial concept, the fabrication of CMUTs has been proposed by a fabrication process in which Silicon-on-Insulator (SOI) wafers were subjected to initial cleaning, after which a 10 nm seed layer of chromium is then deposited thereon using RF-magnetron sputtering to provide an adhesion layer. Following the deposition of the chromium adhesion layer, a 200 nm thick gold layer is next deposited using conventional CMUT deposition processes.
After gold layer deposition, a thin layer of AZ4620 photoresist is spin-deposited on the gold layer, patterned and etched. The gold layer is then etched by submerging the wafer in a potassium iodine solution, followed by etching of the chromium seed layer in a dilute aqua regia, and thereafter rinsing.
The device layer is thereafter etched further to provide acoustical ports for static pressure equalization within the diaphragm, and allowing for SiO2 removal during a release stage.
A top SOI wafer is etched using a Bosch process deep reactive ion etch (DRIE) in an inductively coupled plasma reactive ion etcher (ICP-RIE). After metal etching with the Bosch and DRIE etch, the remaining photoresist is removed by O2 ashing processing. Bosch etched wafer is submerged in a buffer oxide etch (BOE) solution to selectively etch SiO2 without significantly etching single crystal silicon to release the selective diaphragms. Following etching and rinsing, the sensing surfaces (dyes) for each of the arrays are assembled in a system-on-chip fabrication and bonded using conductive adhesive epoxy.
The applicant has appreciated however, existing processes for the fabrication of capacitive micromachined ultrasonic transducers require precise manufacturing tolerances. As a result, the production of arrays of CMUT sensors or transducers on a commercial scale has yet to receive widespread penetration in the marketplace.